Referring to FIGS. 1 and 1a, a center of turning 10 of a trailer 12 is the point about which a trailer will readily turn when pushed or pulled under nominal operations. For most traditional single-axle automotive trailers with a tow bar rigidly attached to the trailer and pivot attachment about the tow vehicle hitch 14, the center of turning 10 is at the midpoint between the trailer's wheels. For a dual axle trailer as shown in FIGS. 1 and 1a, the center of turning 10 is between the wheels in both lateral and longitudinal directions. During nominal operation, the pitch and yaw inertia of this type trailer are accelerated about the center of turning of the trailer. Such acceleration of these pitch and yaw inertias develops reaction loads on the tow vehicle hitch that are transmitted to the tow vehicle, causing it to feel unsteady at times, and may be potentially dangerous. Further, such yaw inertias and reaction loads subject suspension components, steering components and tires of the tow vehicle to additional or undue wear. These accelerated pitch and yaw forces are proportional to the distance between the center of turning of the trailer and the tow vehicle hitch and the magnitude of the inertias. When the center of mass 16 of trailer 12 and its cargo is displaced off the center of turning 10, as may be common in many operations, and when trailer 12 is subjected to the road imperfections, wind and weather conditions, the single pivot trailer imparts additional lateral, or side loads, at the hitch and receiver connection, exacerbating wear and safety considerations.
Two-axle, tow bar-steered agricultural wagons as shown in FIGS. 2 and 3 have been a fixture on the rural American landscape for generations. These trailers can be functionally classified into two groups. As shown in FIG. 2, steered-axle trailers have a pivot 18 between the front wheels, the pivot connected to towbar 20 such that when the towbar is moved to one side or the other, the wheels and axle are turned about the pivot along with the towbar. As shown in FIG. 3, the other type farm trailer is provided with a pivot 22 on or near the front axle 24 between the front wheels, with a towbar 26 connected between pivot 22 and hitch 28 on the towing vehicle. Pivoting connecting rods 30 are connected on towbar 26 just forward of pivot 22, and extend and pivotally connect to a respective steering arm 32 attached to a respective wheel, the wheels conventionally supported for horizontal rotation about a generally vertical kingpin, ball joints or the like (not shown). As such, when towbar 26 is moved to one side or the other, connecting rods 30 pull or push the wheels to rotate about the kingpin, turning the front wheels to steer the wagon according to sideways movement or displacement of the towbar. It is noted that this type steering is called parallel steering, because the steering arms 32 are angled to maintain the wheels in parallel relation at al times. Children's toy wagons and gardening wagons may employ similar construction. Because these trailers cannot be backed up due to a steering instability, they are not generally considered road worthy.
Most modern vehicles and trailers that actively steer their wheels, as seen in FIGS. 4 and 4a, employ steering geometry generally known as Ackerman steering. This type of steering accommodates the facts that, during a turn, the outer wheel travels a further distance than the inside wheel. The implication of this is that, in order for both the inside wheel and outside wheel to track properly, they must be turned to different angles. In other words, for any given turn a vehicle takes, the outside wheel will be turned, with respect to a straight ahead steering position, less than the inside wheel. This is shown in FIG. 4, wherein steering arms 34 are angled such that they each point to a common point 36 generally centered on the rear axle, as shown by dashed lines. This geometry ensures that, for any given turn, such as shown in FIG. 4a, there will be a common point 38 at which the rear axle 40 and front axles 42, 44 will be directed, as shown by dashed lines. Since common point 38 is the center about which the vehicle turns, and the wheel axles all point directly at point 38, it is thus ensured that, since the wheels are perpendicular to their respective axles, the wheels accurately track around the turn without scrubbing, which would otherwise cause excessive wear to the tires and vehicle components.
For anyone who owns a recreational boat, vehicle or the like transported by a trailer to where the vehicle or boat is used, the problems of backing a trailer are well known. The single or double axle trailer as shown in FIGS. 1 and 1a becomes a vehicle steered by the rigidly attached towbar, which is attached to the towing vehicle via a hitch that allows the towbar to pivot from side to side, and also accommodates some vertical pivoting. To back a trailer, the driver is required to steer in an opposite direction to what he would normally steer when travelling forward, with the added length of a towbar magnifying any errors he might make. For a simple single or double axle trailer, the rule of thumb is for the driver to steer with his hands on the bottom of the steering wheel, and move the wheel in the direction he wants the rear of the trailer to go. While this can be difficult, it can be mastered with practice. However, for trailers such as a farm trailer as described above where the front wheels are steered by the towbar, backing of such a trailer can be almost impossible.
Attempts have been made to overcome this problem. In U.S. Pat. No. 4,208,063, to Baker, a trailer is disclosed that comprises a sub-frame for each steered axle that rotates based on an articulation angle between trailer hitch and receiver when the trailer is backed. When towed in a forward direction, no attempt to steer the trailer wheels is made. Drawbacks of Baker are that the tow vehicle hitch must be modified, and the trailer cannot be used with an unmodified tow vehicle.
U.S. Pat. No. 4,824,135, to McGregor, discloses a trailer with a steering system coupled to a tow vehicle steering mechanism about a single pivot at the hitch and receiver connection. In one embodiment, the rear axle of a dual axle trailer is connected to the tow vehicle steering mechanism, while the wheels of the steered front axle are connected to each other through a tow rod, but not with active control. McGregor also discloses a vertically pivoting hitch for accommodating pitching motions of the trailer, but the pivot is at the bumper. Here, pitching motions of the trailer impart large forces to the bumper of the tow vehicle. Like Baker, the hitch and tow vehicle must be modified, and the towed trailer cannot be towed by an unmodified tow vehicle.
U.S. Pat. No. 5,244,226, to Bergh, discloses a single steered-axle trailer with a single pivot at the tow vehicle hitch/receiver that steers opposite the articulation angle between tow vehicle and trailer when in reverse mode and steers at the same angle when in forward mode. However, Bergh carries only one pivot, at the hitch/receiver, and one steered axle. While Bergh improves reverse steering performance for a single axle trailer, the addition of a second axle would cause excessive wheel scrubbing during operation. Also, as discussed above, the hitch and tow vehicle of Bergh must be modified, and the trailer thereof cannot be towed by an unmodified hitch on another vehicle.
In each of Baker, McGregor and Bergh, the center of turning of their respective trailers is at the center between the wheels, with pitch and yaw reaction loads proportional to the distance between the center of turning and the tow vehicle hitch, mass and inertia concerns. As the capacity and size of the trailer increases, there is a corresponding requirement for a larger tow vehicle to stabilize the pitch and yaw reaction loads in addition to providing sufficient pulling and braking forces.
Bartel 2011/042154 discloses a system and method for controlling a trailer whereby the trailer uses a load sensor to signal a microprocessor that automatically signals propulsion and braking commands on a trailer. However, without actively controlled steering, uneven application of drive torque at the trailer wheel would induce large lateral reaction forces at the trailer hitch/receiver. With large vertical and lateral side forces intermixed with tension and compression control forces at said load sensor for drive commands, automatic control of drive system and braking based on load sensor input is limited.
In view of the foregoing, it is apparent that a need exists for a trailer steering system that will allow a steered trailer to be easily backed, and which uses a conventional ball hitch or other conventional hitch, and which in some embodiments requires little to no additional modifications to a tow vehicle or to a tow vehicle hitch, and reduces or eliminates side forces imparted on the tow vehicle hitch and provides for improved automatic control of trailer propulsion and braking functions.